1. Field of the Invention
The present invention relates to a plasma processing apparatus which applies a plasma processing on a substrate such as a semiconductor substrate.
2. Description of the Related Art
In a semiconductor device manufacturing process, a plasma processing such as etching, sputtering or CVD (chemical vapor deposition) has been conventionally heavily used for a semiconductor wafer which is a substrate to be processed.
As a plasma processing apparatus which performs such a plasma processing, various kinds of apparatuses are used, but a capacitive coupling type parallel plate plasma processing apparatus (which will be referred to as a plasma processing apparatus hereinafter) is used in particular.
In this plasma processing apparatus, a pair of parallel plate electrodes (upper and lower electrodes) are arranged in a chamber, an exhaust system is connected, and a high-frequency power supply is connected to the parallel plate electrodes. This plasma processing apparatus brings a process gas into the chamber in which a vacuum is formed by exhaust, a high-frequency voltage is applied to at least one of the electrodes in order to form a high-frequency electric field between the electrodes, a plasma having a process gas atmosphere is formed by this high-frequency electric field, thereby performing a plasma processing with respect to a semiconductor wafer.
In the etching of a film formed on the semiconductor wafer, e.g., an oxide film by using such a capacitive coupling type parallel plate plasma processing apparatus, a low vacuum is formed in the chamber, and a plasma with a medium density is formed, thus enabling preferable to a radical control. Based on this, an appropriate plasma state can be obtained, and hence highly stable and reproducible etching can be realized with a high selectivity.
In recent years, refinement of design rules in the ULSI has further advanced, and a higher aspect ratio of a hole shape has been demanded. However, conventional etching conditions and oxide films, etc. do not satisfy this requirement.
Thus, an attempt has been made to increase the frequency of high-frequency power applied to the above-described parallel plate electrodes and form a high-density plasma while maintaining an excellent plasma dissociation state. Since an adequate plasma can be consequently formed under conditions of a lower voltage, it is possible to adequately cope with further refinement of the design rules.
Meanwhile, according to studies conducted by the present inventor, since the upper electrode is formed of a conductor or a semiconductor in plasma processing apparatuses, it was found that the following disadvantages are generated.
As described above, when an application frequency is increased in order to form the high-density plasma, the inductance on the electrode surface to which a high frequency is applied cannot be ignored, the electric field becomes stronger at the center of the electrode, and the electric field distribution in the radial direction becomes uneven. When the electric field distribution becomes uneven, the plasma density becomes uneven, and the etching rate distribution also becomes uneven. Therefore, it is required to eliminate factors which produce unevenness of the electric field distribution, and thus make the etching rate distribution even.
However, problems in the conventional method using high-density plasma are not necessarily clearly recognized, and the attempt to eliminate unevenness of the electric field distribution is insufficient at the plasma processing.